1. Field of the Invention
The invention relates to a device for laterally coupling pump light into a fiber. The device comprises the fiber which runs in a longitudinal direction and which has an optically active medium to be pumped, with a lateral wall running approximately in the longitudinal direction, and at least one light coupling means, at least some part of which runs parallel to the fiber along the lateral wall.
2. Description of Related Art
Usually, fiber lasers or fiber amplifiers are optically pumped in that pump light is radiated into the fiber via a facet of the optically active fiber. For this purpose, the pump light must be so directed or focussed onto the facet of the fiber that it enters into the pump core (inner cladding) of the fiber and creates a population inversion in the optically active signal core of the fiber which is surrounded by the pump core (inner cladding). However, a coupling-in of pump light via a facet of the fiber can be difficult due to the small cross-sectional area of the pump core, which requires a precise adjustment of the pump beam. Furthermore, high field strengths can occur in the coupling area which can damage the fiber. A coupling-in via a facet of the fiber can also be problematic since the pump light must be suitably superposed, for which purpose various precautions such as the superposition of different beam polarisations or wavelengths can be necessary. Also, a highly inhomogeneous distribution of pump light within the fiber can result through coupling via the facet.
Optical fibers usually have a round cross-section. However, there are also fibers with at least one flat lateral wall and fiber with a “flat”, for example with octagonal cross-section or similar. A fiber in the sense of the present disclosure includes fibers with different cross-sectional forms.
Due to these problems, the lateral coupling of pump light into the fiber has already been suggested. In the publication DE 10 2011 103 286, the transverse coupling of pump light into the fiber is described, wherein the pump beam strikes the fiber perpendicularly through refocusing in an elliptical pump chamber. This represents the state of the art for flashbulb-pumped solid state lasers. However, in this case the coupling efficiency is often unsatisfactory.
According to the publication U.S. Pat. No. 6,477,295 light is coupled transversely into a fiber in that pump light is first radiated into a glass cladding surrounding the stripped fiber, the refractive index of which is matched to the refractive index of the fiber cladding (see FIGS. 8 and 10). Due to the large surface of the glass cladding, high field strengths at the boundary surface between glass and air such as can occur with the coupling-in of pump light via the fiber facet are avoided and the destruction threshold of the pump arrangement is increased. On the other hand, this method of coupling leads to an inhomogeneous pump light distribution within the pump core of the optical fiber. The same applies to coupling-in at a bend in the fiber (see FIG. 6).